In the field of genetic engineering, precise control of gene expression is an invaluable tool for studying, manipulating and controlling development and other physiological processes. For example applications for regulated gene expression in mammalian systems include inducible gene targeting, overexpression of toxic and teratogenic genes, anti-sense RNA expression, and gene therapy (see, for example, Jaenisch, R. (1988) Science 240, 1468-1474). For cultured cells, glucocorticoids and other steroids have been used to induce the expression of a desired gene.
As another means for controlling gene expression in mammalian systems, an inducible tetracycline regulated system has been devised and utilized in transgenic mice, whereby gene activity is induced in the absence of the antibiotic and repressed in its presence (see, e.g, Gossen et al. (1992) PNAS 89, 5547-5551; Gossen et al.(1993) TIBS 18, 471-475; Furth et al. (1994) PNAS 91 9302-9306; and Shockett et al. (1995) PNAS 92, 6522-6526). However, disadvantages of the inducible tetracycline system include the requirement for continuous administration of tetracycline to repress expression and the slow clearance of antibiotic from bone, which interferes with regulation of gene expression. While this system has been improved by the recent identification of a mutant tetracycline repressor which acts conversely as an inducible activator, the pharmacokinetics of tetracycline may hinder its use during development when a precise and efficient "on-off" switch is essential (see, e.g., Gossen et al. (1995) Science 268, 1766-1769).
Accordingly, there is a need in the art for improved systems to precisely modulate the expression of exogenous genes in mammalian subjects. For example, a non-mammalian-based transcription regulating system would be extremely desirable for general application to transgene regulation in in vitro, ex vivo, and in vivo applications, as well as transgenic animals. A system that is simple, compact and dependent on ligands which are relatively inexpensive, readily available and of low toxicity in animals would prove useful for stimulation of regulated systems.